The type of Russia military aircraft that violated Estonian airspace earlier this week is used for electronic spying, says Estonian Air Force Lieutenant Colonel Janek Lehiste.
The IL-20 was in Estonian airspace for around one minute on Tuesday. The next day, Estonia summoned Russian Ambassador Yuri Merzlyakov and delivered a note of protest.
NATO jet fighters were also scrambled and made visual contact.
Lehiste said NATO jets quickly identified the aircraft, which had no fight plan, had switched off its transponder and avoided radio contact with Estonian air traffic controllers.
He said the IL-20 aircraft listen to everything in a magnetic field.
Mati Raidma, head of Parliament's National Defense Committee said the aim was to see how information is swapped between Baltic Sea nations, and who is communicating with whom.
The IL-20 took off from Kaliningrad and headed towards Denmark, but was intercepted by Danish fighters, after which it turned towards Sweden. After Swedish jets were scrambled the aircraft turned towards Estonia.
Russia has denied violating Estonia's airspace.